Disclaimer: I am not paid or endorsed by Sigma and these comments are my own only.

In the specs, the 135mm has most of the advantages. It's smaller, lighter, focuses closer, and costs less. The 105mm has an extra 2/3 stops, but at an extra pound of weight and the size not actually fitting in my bag.. those are big drawbacks. Since this is an Art lens, it can also me calibrated further with the Sigma dock. But how does the picture quality compare? For the purposes of this test, I don't care about shutter speed and ISO, I only care about how the extra two open stops and shorter compression compare with the bokeh on the 135mm. Which means I'll be shooting wide open on both lenses.

The 105mm will always be on the left in these images.

I began thinking I'd show the difference in the two by switching back and forth for each shot. I tried to set up the shot as closely as possible to each other.

So first up I decided to shoot my trusty somewhat mangled birdfeeders. There's a house across the street behind them. The 105mm shows me less of the road and the house, so that's a bonus.

Moving object test was quite nice, the 105 handles quick focusing with ease. One thing I've noticed that is that it doesn't focus as closely as I'd like. I'd get my shot composed, then have to back up because it wasn't in focus.

Outside, the differences are less obvious.

The problem with doing the switch between them outside is that I was alone and juggling the two of them meant I was sure to drop one, so I just forgot about that and shot some things that I wanted with the 105 until I had someone to help hold the lenses as I switched them.

Overall, it's a solid lens, exactly what I would expect from Sigma, and since both of these lenses are Sigma Art line lenses, I expect both of them to be utterly fabulous. It handles very quickly, but that extra weight was a bummer. As it was really overcast out, the extra two stops of light was lovely. Next up, shooting with a human subject to see how it handles focusing and bokeh.

So in this pair, we can see that the 105mm on the left is slightly less yellow, which I find interesting. I wanted to shoot this specific location because I wanted to see how it handled the framing foreground elements. For me, the 135mm compresses this a bit too much and makes it look messy. The 105mm on the left is cleaner and more separated.

The bokeh in all of those on the left is also nicer for me as well, just a bit more out of focus. Now obviously if I were shooting at 1.8 on both lenses, that might not be the case due to the shorter compression on the 105mm.

This one for me is where the 105mm clearly begins to stand out as less messy and cluttered. Look at the bushes behind her. They don't compete with her quite as much. Her face is also a little bit less round.

The foreground in this one is again less messy. I'm not really sure a client would notice the difference. Is it worth the extra weight, size, and price? For me, the answer is YES. I really enjoyed just that extra bit of magic. I'll end up trading my Sigma 135mm 1.8 Art for the Sigma 105mm 1.4 Art. I'll have to cram it into my bag or whatever.